94 TORREYA 



It would be possible to proceed at great length with the story of how false 

 blossom virus upsets growth relationships, enlarges, distorts, and malforms 

 flowers, and transforms plants of different species to such an extent that they 

 can scarcely be recognized. But there would be no point in doing this, for the 

 variability in the symptoms produced in different species is almost endless. It 

 perhaps is sufficient to say that in all plants to which false blossom virus was 

 taken it caused virescence and gigantism in flowers or parts of flowers, 

 chlorosis and dwarfing in leaves, and an elongating or a shortening of inter- 

 nodes of stems. 



Another effect of certain viruses on growth in plants that needs to be men- 

 tioned is repression of dormancy and maturity. Many biennial and perennial 

 plants pass the winter in a dormant state. When brought into a greenhouse 

 where good growing conditions are maintained and where they might be ex- 

 pected to grow continuously, they become dormant or semi-dormant as the 

 winter season approaches. Many annual plants grow to maturity in a few 

 months and then die. When affected by certain virus diseases, perennial plants 

 fail to go into a dormant state regardless of environmental conditions. Peach 

 trees affected by yellows disease do not stop growing as cold weather comes 

 on but continue vegetative growth until the tender tips of branches are frozen 

 and killed. 



China aster plants set in the field late in May or early in June blossom in 

 August and mature seeds in early autumn. By the time cold weather arrives, 

 all healthy plants have died. The course of events is very different for plants 

 that contract the aster yellows disease. They produce malformed virescent 

 flowers and sterile seeds (5). It is true that some diseased plants produce 

 viable seeds, but such seeds are borne only by flowers that have not been in- 

 vaded by the virus. Instead of affected plants maturing and dying as cool 

 weather approaches, they live and grow. They of course do not grow very fast 

 and are eventually killed by low temperatures. But there is a period during 

 which the only living plants in the field are those affected by the aster yellows 

 disease. It is thus clear that the virus lengthens the life of the plant. 



When healthy potato plants are grown in greenhouses, they produce tubers, 

 mature, and die. If infected by the witches' broom virus, they produce tubers 

 but they do not mature and die. Growth continues summer and winter for an 

 indefinite period of time. There is a potted potato plant with witches' broom 

 in one of our greenhouses that has been growing there for more than two years. 

 Its healthy sister plants matured and died long ago. The witches' broom virus 

 has had a favorable effect on the longevity of the plant. 



Certain plant virus diseases are readily cured by heat. When affected plants 

 are held at moderately high temperatures for appropriate periods of time, the 

 viruses that cause these diseases are inactivated but the plants are not seriously 



